Is war between the Sudans inevitable?
Sudan and South Sudan are at war - of sorts - and have been for longer than it appears.
President Omar al-Bashir's bellicose declarations about liberating South Sudan from the "insects" who govern it stole the headlines.
South Sudan's conquest of the Heglig oilfields last week, with its far-reaching military and economic implications, is the physical proof of the conflict.
But it is not yet certain these recent events will result in meaningful and continued fighting outside the already embroiled border region.
President Bashir delivers two styles of speech.
In the first he reads from a measured text.
In the second he speaks off the cuff, often using humour, always stirring the passion of his supporters with dramatic rhetoric and images.
In the past he has talked of an Islamic Sudan with "no place for ethnic and cultural diversity" and hinted at painful events that might befall elections observers, UN peacekeepers or other enemies.
These kind of speeches energise the base, as they say in American politics - but do not always translate into a meaningful change to an established reality.
His fiery words in Khartoum and then el-Obeid, in front of military recruits wearing combat uniforms, may well fall into this category.Land swap?
After all, it is possible to argue Sudan and South Sudan are already fighting an undeclared war, and have been for some time.
Main disputes between the two Sudans
- The amount the South should pay Sudan to use its oil pipelines
- Demarcating the border
- Both sides claim Abyei
- The rights of each other's citizens now in a foreign country - there are estimated to be 500,000 southerners in Sudan and 80,000 Sudanese in the South
- Each accuses the other of supporting rebel groups on its territory
In May last year, before South Sudan became independent, Sudanese tanks rolled into the disputed region of Abyei, sweeping aside South Sudanese police, even though these men were probably soldiers put in a different coloured uniform.
Khartoum's "invasion" of Abyei was condemned initially, then in effect accepted as a reality on the ground, to factor into negotiations.
Juba may have been given a handy precedent for its recent takeover of neighbouring Heglig.
Already the idea of an Abyei-Heglig swap has been hinted at.
But Abyei is not the only example which suggests a long-running state of war between the neighbours.
Since rebels took up arms against Khartoum in South Kordofan, and then Blue Nile, there have been accusations they are being supported by Juba.
The SPLM-North rebels once fought alongside the men who won independence for South Sudan, but were left north of the border at separation.
The US and the UN, among others, have told Juba to stop supporting SPLM-North - an accusation South Sudan denies.
Khartoum has its own difficult-to-avoid charges to ward off too: South Sudanese rebels are largely based in the Sudanese capital.
International weapons monitoring group Small Arms Survey has shown, convincingly, that Juba and Khartoum are both almost certainly supporting proxy rebels on the other's territory.Oil output hit
It has been a key and heated topic of the negotiations in Ethiopia's capital, Addis Ababa, which are on hold for the moment.
Khartoum perceives the well-armed SPLM-North's actions as proof that South Sudan has been attacking it for many months. Juba has seen ethnic violence fuelled over decades by Khartoum's divide and rule tactics.
There have also been direct Sudan-South Sudan clashes on the border before these latest events, and Sudan has been caught bombing South Sudanese territory too.
Under this reading, Sudan and South Sudan have been carrying out a sort of low-level, unacknowledged war for some time.
There is no doubt Heglig has taken the situation to a much more dangerous dimension.
South Sudan's troops have struck a crippling blow to Sudan's economy.
An oil industry source, who works in Heglig, says it may account for as much as 80,000 barrels per day of Sudan's 120,000-130,000 bpd output.
Khartoum cannot afford to lose this. It simply does not have the foreign currency reserves to keep importing fuel to make up the gap, and there are fears the prices of basic good could rise.
Many of the South Sudanese leaders, and the various Sudanese rebel groups with similar agendas, will hope this pressure could bring about the downfall of President Bashir.
In this context it is evident the Sudanese have to regain the oilfields, either through an South Sudanese withdrawal because of international pressure, or through force.
Or, as a Sudanese official put it, "by hook or by crook".Military limitations
It is also easy to see why President Bashir might favour at least a limited dose of military action.
He and others in his inner circle may hope the unifying nature of combat against a "common enemy" would rally a fractured nation behind him.
It may take more than this, because after two decades in power, in which the Sudanese have been constantly at war, the people's patience is running out.
South Sudan has its problems too.
The decision to shut down oil production in January - 98% of government revenue - after Sudan impounded South Sudan's oil shipments amid a dispute over transit fees - was highly popular, but the effects are being felt.
There is the beginnings of a fuel crisis in Juba, and infrastructure projects and development are on hold.
Here, too, President Salva Kiir may feel he can stave off criticism by the patriotic fervour created by fighting a war.
Both sides are mobilising troops.
Further fighting in Heglig is inevitable, and if Sudan retakes the oilfields, it is difficult to tell whether its soldiers would stop there.
The question is whether either side really intends - or is able - to spread the war further.
These have been skirmishes in a couple of locations, but no sign yet of major battles.
Both countries have serious military limitations.
Sudan is fighting off rebels in Darfur, South Kordofan and Blue Nile, and was not able to hold on to its prize possession, Heglig.
South Sudan does not have the firepower, particularly in the air, of its old enemies.
The lack of oil revenue also means both would struggle to pay for another costly war.
Both armed forces, and in particular South Sudan, have command and control problems, which means on a local level military leaders sometimes launch assaults which surprise the top brass in the capital.
An all-out war would create great misery, as wars do.
But it is not yet clear if either side has the resources, or the desire, to conduct a war for a sustained period of time outside of a relatively limited front.
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. The residents of war-affected Darfur and South Sudan are still greatly dependent on food aid. Far more than in northern states, which tend to be wealthier, more urbanised and less reliant on agriculture.